Fiber optic sensing systems may be used in performing various downhole monitoring and measurements, for example, quantification of vibrations, acoustics, pressure, and temperature changes. Particular types of such systems may include distributed acoustic sensing (DAS) and time-domain multiplexed optical interferometry (TDM). Current fiber optic systems may employ a light source, fiberoptic coupler, and multiple detectors.
The light source generates a highly coherent optical light along an optical fiber. As the light travels along the optical fiber, it may reflect back towards the source due to impurities of the fiber, also known as “Rayleigh backscatter,” or may be purposefully reflected via reflectors, such as fiber Bragg gratings (FBG's). The reflected signals may be received by the fiberoptic coupler, which then generates and conveys variations of the signal to the multiple detectors. The detectors may then generate outputs to an analog-to-digital converter (ADC) for quantification of each received signal, wherein a computer utilizes the digitized signals to calculate an optical signal phase change. The calculated signal phase change corresponds to changes in a downhole parameter of interest.
One problem with current systems is saturation of the optical receiver(s) or subsequent analog to digital converters, which is a combined result of increased amplification to overcome attenuation and a large common mode signal component which consumes a large fraction of the optical receiver's and ADC's dynamic range and contributes noise to the optical phase measurements. The increased noise and inadvertent saturation may lead to incorrect phase measurements.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.